This invention relates to a digital radio receiver for receiving an electric wave from an artificial satellite (that may be called a xe2x80x9csatellite wavexe2x80x9d) or an electric wave on the ground (that may be called a xe2x80x9cground wavexe2x80x9d) to listen in a digital radio broadcasting and, in particular, to a loop antenna for use in the digital radio receiver.
In recent years, a digital radio receiver, which receives the satellite wave or the ground wave to listen in the digital radio broadcasting, has been developed and is put to practical use in the United States of America. The digital radio receiver is mounted on a mobile station such as an automobile and can receive an electric wave having a frequency of about 2.338 gigahelts (GHz) to listen in a radio broadcasting. That is, the digital radio receiver is a radio receiver which can listen in a mobile broadcasting. In addition, the ground wave is an electric wave in which a signal where the satellite wave is received in an earth station is frequently shifted a little.
In order to receive such an electric wave having the frequency of about 2.338 GHz, it is necessary to set up an antenna outside the automobile. Although a variety of antennas having various structures have been proposed, the antennas of cylindrical-type are generally used rather than those of planer-type (plane-type). This is because a wider directivity is achieved by forming the antenna into a cylindrical shape.
A loop antenna is known in the art as one of the antennas of the cylindrical-type. The loop antenna has structure where one antenna lead member is wound around a peripheral surface of a hollow or solid cylindrical (which is collectively called xe2x80x9ccylindricalxe2x80x9d) member in a loop fashion, namely, is an antenna having the form of a loop. The cylindrical member may be merely called a xe2x80x9cbobbinxe2x80x9d or a xe2x80x9cdielectric corexe2x80x9d in the art. In addition, the antenna lead member may be merely called a xe2x80x9clead.xe2x80x9d It is known in the art that the loop antenna acts as an antenna having a directivity in a longitudinal direction thereof if the antenna lead member has an all around length which is selected to about one wavelength. This is because the antenna lead member has a sinusoidal distribution of a current.
Although it is necessary for the loop antenna to feed to it, a four-point feeding is generally adopted to the loop antenna. In order to receive circular polarization, feeding is carried out at four points having a phase difference of 90 degrees from one another. The loop antenna with the four-point feeding is called in the art a four-point feeding loop antenna. In a conventional four-point feeding loop antenna, a feeding is directly carried out to a loop portion.
More specifically, the conventional four-point loop antenna comprises a cylindrical body formed by rounding a flexible insulation film around a central axis in a cylindrical fashion, a loop portion made of conductor that is formed on the cylindrical body along a peripheral surface thereof around the central axis in a loop fashion, and four feeders formed on the peripheral surface of the cylindrical body to feed the loop portion at four points. The loop portion is directly connected with each of the four feeders.
After the electric wave is received by the loop portion as a received wave, the received wave is divided through the four feeders into four partial received waves which are phase shifted and combined by a phase shifter so as to match phases of the four partial received waves to obtain a combined wave, and then the combined wave is amplified by a low-noise amplifier (LNA) to obtain an amplified wave which is delivered to a receiver body. A combination of the four-point feeding loop antenna, the phase shifter, and the low-noise amplifier is called an antenna unit.
In the manner which is described above, inasmuch as the conventional four-point feeding loop antenna directly feeds the loop portion from the four feeders, the conventional four-point feeding loop antenna is disadvantageous in that it has a too high feeding impedance. Thus, the conventional four-point feeding loop antenna is disadvantageous in that it is difficult to obtain an impedance match.
It is therefore an object of the present invention to provide a four-point feeding loop antenna which is capable of easily obtain an impedance match.
Other objects of this invention will become clear as the description proceeds.
According to an aspect of this invention, an electromagnetic coupling type four-point feeding loop antenna comprises a cylindrical body formed by rounding a flexible insulator film member around a central axis in a cylindrical fashion. The cylindrical body has a peripheral surface. Made of conductor, a loop portion is formed on the cylindrical body along the peripheral surface around the central axis in a loop fashion. Four feeders are formed on the peripheral surface of the cylindrical body to feed to the loop portion at four points. The electromagnetic coupling type four-point feeding loop antenna has gaps between the loop portion and the four feeders, thereby feeding to the loop portion by electromagnetic coupling.
In the above-mentioned electromagnetic coupling type four-point feeding loop antenna, the flexible insulator film member may substantially have a rectangular shape having an upper side, a lower side, a first lateral side, and a second lateral side. In this event, the cylindrical body is formed by connecting the first lateral side with the second lateral side. The loop portion may be formed on one surface of the flexible insulator film member in the vicinity of the upper side.
Each of the four feeders may extend on the flexible insulator film member from the lower side to the vicinity of said loop portion. In this event, the loop portion is connected with four electromagnetic coupling wires extending on the flexible insulator film member from the loop portion along the four feeders toward the lower side with the gaps left between the four feeders and the four electromagnetic coupling wires, respectively.
Alternatively, each of the four feeders may comprise a main feeding wire portion extending on the flexible insulator film member from the lower side to the vicinity of the loop portion and a sub-feeding wire portion extending on the flexible insulator film member in parallel with the loop portion from a tip of the main feeding wire portion with the gap left between the loop potion and the sub-feeding wire portion.